The invention relates generally to inorganic membranes such as thin layers, films and coatings of zeolite material and more particularly to inorganic membranes that include cations in the zeolite pore structure.
Zeolites are crystalline aluminosilicates, containing molecular sized pores. They are an important material for catalyzing reactions, and are commonly used in the catalytic cracking of petroleum and other reactions of hydrocarbons. For example, ZSM-5 zeolites have an MFI framework with intersecting pores having a diameter of 5 to 6 .ANG.. Zeolites are commonly provided in a powder form and are used to selectively crack straight chain molecules over branched chain molecules. In addition, zeolite particles have been used to selectively form para-xylene, which is used to make terephthalic acid for various polymerization reactions and in the synthesis or rearrangement of xylenes.
Zeolites are formed with a three dimensional structure in which the tetrahedra of primarily SiO.sub.4 and AlO.sub.4 are crosslinked by sharing oxygen atoms, whereby the ratio of Si to 0 atoms is 1:2. Depending on the manner of bonding, the crystal structures can have various ring sizes containing 8, 10 or 12 metal/semimetal atoms. This presence of rings provides a crystal structure with pores and chambers that are of molecular size. The presence of these chambers promotes the separation of molecules according to their size and their affinity to the zeolitic surface. Because zeolites are commonly available in the form of fine powders, catalytic systems including zeolites frequently use packed or fluidized beds made of zeolite crystal powders or small pellets made up of zeolite crystals. The reactants are fed through the bed, generally at relatively high temperatures, and the products are collected at the other end. However, it is very possible for molecules to pass through the bed without ever entering a zeolite pore. Thus, a significant portion of the reactants will pass through the bed and remain unreacted. Furthermore, the small size of the materials make it difficult to handle.
Various attempts have been made to overcome these problems. For example, U.S. Pat. No. 4,740,219, the contents of which are incorporated herein by reference, discusses composite membranes of zeolite particles on organic membranes. Unfortunately, such membranes were not fully suitable and for example, lack sufficient temperature stability. U.S. Pat. Nos. 5,019,263 and 5,069,794 report the formation of membranes formed with layers of zeolitic material. The contents of these two patents are incorporated herein by reference. Sano, et al., Bull Chem. Soc. Jpn. 65:146 (1992) and Sano, et al., Zeolites, 11:842 (1991) discuss forming layers of zeolite material. See, also the methods discussed in U.S. Pat. No. 4,740,219 Canadian Patent No. 1,235,684, Ishikawa, A., J. Chem. Soc., Chem. Commun., 764 (1989), the contents of which are incorporated herein by reference. However, the methods discussed therein for forming zeolite membranes are not fully satisfactory. Moreover, neither of these patents nor these articles discuss reliable ways of forming catalytic centers by incorporating cations within the zeolite pore structure. Providing zeolite membranes in which cations are present within the pore structure is important for conducting various important chemical reactions.
Accordingly, it is desirable to provide zeolite in a more suitable form to overcome the shortcomings of the prior art.